1. Field of the Invention
This invention relates to a method of and apparatus for heating subterranean formations. In another aspect, this invention relates to an improvement in method and apparatus for recovering a fluid from a subterranean formation by heating.
2. Description of the Prior Art
Uniform heating of a subterranean formation has yet to be achieved in the art. The achievement of this goal has been hindered principally by the fact that one can only enter a formation at discrete points. Thus, limited access to a formation has prevented those skilled in the art from uniformly heating a subterranean formation. The present invention provides a method and apparatus for achieving a more nearly uniform heating of a subterranean formation than was heretofore known.
A wide variety of fluids are recovered from subterranean formations. These fluids range from steam and hot water geothermal wells through molten sulfur to hydrocarbonaceous materials having greater or lesser viscosity. The hydrocarbonaceous materials include such diverse materials as petroleum, or oil; bitumen from tar sands; natural gas; and kerogen, a substance found in oil shales.
The most common and widely sought fluid to be produced from a subterranean formation is petroleum. The petroleum is usually produced from a well or wells drilled into a subterranean formation in which it is found. A well is producing when it is flowing fluids. The words "to produce" are used in oil field terminology to mean to vent, to withdraw, to flow, etc., pertaining to the passage of fluids from the well.
There are many hydrocarbonaceous materials that cannot be produced directly through wells completed within the subterranean formation in which the fluids are found. Some supplemental operation is required for their production. At least three such materials are kerogen in oil shale, bitumen in tar sands, and highly viscous crude oil in oil-containing formations. The first two frequently involve special production problems and require special processing before a useful product can be obtained. These materials have at least one common characteristic, however. That is, heat can bring about the necessary viscosity lowering, with or without conversion of the in situ product, to enable the hydrocarbonaceous material to be produced from its environment.
Several processes supplying heat in situ have been developed in the past. These processes employ so-called in situ combustion, fire flood, stream flood, or similar related recovery techniques in which at least one fluid containing or developing the heat is passed through the formation. Because of "liquid blocking" the usual methods of in situ heating which require injection of a fluid are often ineffective with the three materials discussed previously.
Liquid blocking is simply the building up of a bank of liquid hydrocarbonaceous material and water in advance of the front of the fluid being injected, combustion front, or the like. With this liquid build-up, permeability is dramatically reduced and excessively high pressures become necessary for continued injection at the high rates desired. A wide variety of techniques have been attempted in order to cure, or minimize, this problem; but to date they have not been totally successful.
Regardless of whether or not a fluid is injected into the formation, production is enhanced and liquid blocking minimized if the viscosity of the fluid can be reduced by heating. One of the problems encountered in pre-heating a subterranean formation has been that it tends to channel the heat along crevices or regions of greater permeability to create nonuniform, or extremely variable heating effects that contribute to premature breakthrough of any supplemental recovery operation. Heating more uniformly a subterranean formation containing the fluid not only helps alleviate the problem with liquid blocking, but can convert the liquid block to an asset that will tend to average minor permeability inhomogeneities, achieve increased macroscopic sweep efficiency of any fluid injected and improve the recovery of any such recovery operation subsequently initiated.
Thus, the prior art processes have not been successful in providing method and apparatus for heating a subterranean formation substantially uniformly throughout a predetermined pattern without requiring the injection of one or more fluids for effecting the heating in situ.